<!DOCTYPE html>
<html lang="ch">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .box {
            width: 300px;
            margin: 0 auto;
            /* overflow: hidden; */
        }

        ul {
            list-style: none;
        }

        .box .item {
            float: left;
            width: 148px;
            text-align: center;
            border: 1px solid black;
            background-color: blue;
            color: #fff;
            line-height: 40PX;
        }

        .item:hover {
            color: blue;
            background-color: #fff;
        }

        .item ul {
            display: none;
            background-color: white;
            color: black;
        }

        .item:hover ul {
            display: block;
        }

        .item li:hover {
            color: aqua;
        }
        /* 如果遇到多个嵌套 加上一个>号 */


        .box1{
            height: 200px;
            background-color: yellow;
        }

        .box::after{
            content: "1111";
            clear: both;
            display: block;
            width: 0;
            height: 0;
            visibility: hidden;
        }
    </style>

</head>

<body>

    <ul class=" box">
        <li class="item">视频教程
            <ul>
                <li>111111111</li>
                <li>22222222222</li>
                <li>33333333333</li>
                <li>444444444</li>
                <li>555555555555 </li>
            </ul>

        </li>
        <li class="item">认证考试

            <ul>
                <li>444444444</li>
                <li>555555555555</li>
            </ul>

        </li>
    </ul>


    <div class="box1">11111111111</div>

</body>

</html>